1. Field of the Invention
The present general inventive concept relates to a inkjet printhead and an inkjet image forming apparatus including the inkjet printhead, and more particularly, to a thermally driven inkjet printhead having a heater that allows the inkjet printhead to be driven at a low power and that can increase a lifespan and stability of the inkjet printhead, and an inkjet image forming apparatus including the inkjet printhead.
2. Description of the Related Art
In general, inkjet image forming apparatuses are devices such as printers for printing images having a predetermined color by ejecting a small volume of ink droplets from an inkjet printhead on a desired position of a printing medium. Inkjet image forming apparatuses can be classified into shuttle type inkjet image forming apparatuses, in which a printhead prints an image by traveling in a same direction (hereinafter a secondary ejection direction) and in a perpendicular direction (hereinafter, a primary ejection direction) to the moving direction of a printing medium, and line printing type inkjet image forming apparatuses which have recently been developed for high-speed printing and have an array type inkjet printhead.
The line printing type inkjet image forming apparatus includes one or multiple array type inkjet printheads to dispose a plurality of nozzles to correspond to at least a width of a printing medium. Printing is performed in a state that the inkjet printheads are fixed while the printing medium moves in the secondary ejection direction, thereby enabling high-speed printing.
The inkjet printheads can be classified into two types according to the mechanism by which ink droplets are ejected. A first type is a thermal inkjet printhead that ejects ink droplets by an expansion force of ink bubbles generated in the ink using a heat source, and the second type is a piezoelectric inkjet printhead that uses a piezoelectric element and ejects ink droplets by a pressure applied to the ink due to a deformation of the piezoelectric element.
The mechanism of ejecting ink droplets in the thermal inkjet printhead will now be described in more detail. When a pulse type power is applied to a heater formed of an electrical heating material, the heater is instantaneously heated to approximately 500° C., and ink adjacent to the heater is instantaneously heated to approximately 300° C. Accordingly, the ink boils, and thus, bubbles are generated in the ink. The bubbles expand and apply a pressure to the ink filled in an ink chamber. As a result, the ink around nozzles is ejected to the outside of the ink chamber in the form of droplets through the nozzles.
The thermal inkjet printhead can be further classified into a top-shooting type, a side-shooting type, and a back-shooting type thermal inkjet printhead according to directions of bubbles growing and ink droplet ejection. In a top-shooting type inkjet printhead, bubbles grow in a direction in which ink droplets are ejected. In a side-shooting type inkjet printhead, bubbles grow in a direction perpendicular to the direction in which ink droplets are ejected. In a back-shooting type inkjet printhead, bubbles grow in a direction opposite to the direction ink droplets are ejected.
FIG. 1 illustrates a lateral cross-sectional view of a conventional inkjet printhead. Referring to FIG. 1, the conventional inkjet printhead includes a substrate 11, a chamber layer 20 which is stacked on the substrate 11 and includes an ink chamber 22 in which ink is filled, and a nozzle layer 30 which is stacked on the chamber layer 20 and includes a nozzle 32 through which the ink is ejected. A heater 13 for generating bubbles by heating ink is formed below the ink chamber 22.
An insulating layer 12 for thermally and electrically insulating the heater 13 from the substrate 11 is formed on the substrate 11. The heater 13 can be formed by patterning a thin film deposited on the insulating layer 12 using a material such as TaAl, TaN, HfB2, etc. An electrode 14 for applying power to the heater 13 is formed on the heater 13, and can be formed of a conductive metal such as aluminum.
A passivation layer 15 for protecting the heater 13 and the electrode 14 is formed on surfaces of the heater 13 and the electrode 14. The passivation layer 15 prevents chemical and mechanical corrosion of the heater 13 and the electrode 14 by blocking the heater 13 and the electrode 14 from direct contacting ink, and can be formed of a silicon nitride SiNx having a low thermal conductivity.
An anti-cavitation layer 16 is formed on the passivation layer 15. The anti-cavitation layer 16 protects the heater 13 and the electrode 14 from a cavitation force generated when the bubbles disappear, and can be mainly formed of Ta.
Recently, due to a high integration and a high-speed operation of inkjet printheads, inkjet printheads that can be operated at a low power are required. Low power operation is particularly required in an array type inkjet printhead that has a plurality of nozzles and operates at a high frequency. To realize a low power operation of an inkjet printhead, a high efficiency of the heater 13 is essential.
The heater 13 must be able to instantaneously increase the temperature of ink to more than 300° C. in order to generate bubbles in the ink. However, a conventional inkjet printhead has a structure in which the heater 13 is shielded from ink by layers having a predetermined thickness, such as the passivation layer 15 and the anti-cavitation layer 16. Therefore, to transmit a heat to the ink, an electric energy to be applied to the heater 13 must be increased.
In particular, in an array type inkjet printhead, a large amount of electric energy for driving the heaters is instantaneously consumed since a few tens of thousands of heaters corresponding to the number of nozzles of the array type inkjet printhead are operated at a high frequency for high-speed printing. The inefficiency of the heaters can affect a design limit of circuits and elements, an integration density of the nozzles, or can be a safety issue of a line printing type inkjet image forming apparatus. Also, heat can be accumulated in the inkjet printhead resulting in degradations in physical and chemical properties of the ink, for example, a viscosity, thereby reducing printing quality.
If the passivation layer 15 and the anti-cavitation layer 16 that shield the heater 13 from ink are removed, energy consumption can be reduced, and accordingly, the efficiency of the heater 13 can be increased. However, if the heater 13 formed of TaAl, TaN, or HfB2 directly contacts ink, the heater 13 can be corroded through a reaction with moisture of the ink, which can greatly change the resistance of the heater 13, thereby causing electrical and chemical safety problems with the heater 13. Also, the heater 13 can be damaged by a cavitation force generated when the bubbles disappear, thereby causing a mechanical safety problem.
Therefore, there is a need to develop an inkjet printhead that has no electrical, chemical, and mechanical problems when the heater 13 directly contacts the ink, without the requirement for the passivation layer 15 and the anti-cavitation layer 16.